Forty-three years have elapsed since T. H. Maiman reported the first successful operation of a laser (the pulsed ruby laser). During this long period of time, dozens of new types of lasers were subsequently discovered (e.g. pulsed and CW gas-discharge lasers, pulsed and CW optically-pumped solid-state and liquid-state lasers, semiconductor diode lasers, etc.) and were found to have technically sophisticated but vital uses in a wide variety of fields (e.g. in medicine, manufacturing, basic scientific research, communications, computers, consumer products, military devices, etc.). All of these lasers operate on the same basic principle outlined in three fundamental U.S. laser patents (U.S. Pat. No. 2,929,922 by C. H. Townes & A. L. Schawlow and U.S. Pat. Nos. 4,053,845 and 4,704,583 by G. Gould), namely, that laser emission results from stimulated emission occurring on an optical transition that is inverted, i.e. that has more active atoms (ions, molecules, etc.) in the upper level than in the lower one. However, beginning about the year 1976, scientists working in the field of quantum electronics began intensively investigating both theoretically and experimentally the striking properties of a gas of atoms (ions, molecules, etc.) simultaneously saturated on two electric-dipole-allowed transitions sharing a common level. It was discovered that all atoms in such a prepared system become coherently phased by the action of the two applied monochromatic resonant laser beams. It is now customary to refer to such atoms as being “dressed” by the photons of the two applied laser beams. Such a gas of coherently phased (i.e. “dressed”) atoms displays a number of highly unusual properties, the most well known and remarkable constituting a condition of complete transparency at both applied monochromatic laser beam frequencies. This is the effect that is usually termed “Electromagnetically Induced Transparency (EIT)”. The main thrust in a sizeable portion of the aforementioned “dressed atom” studies became to try to discover ways in which such systems could be utilized to generate coherent light beams without population inversions being present on the laser transitions. A number of interesting proposals for “lasers without inversion (LWI)” were made, and relevant experiments were conducted. Although successful CW LWI operation based upon dressed-atom gain media was technically achieved in a sparse handful of tour-de-force efforts—the most notable being those reported by Zibrov et al. in Physical Review Letters 75, 1499 (1995) and by Padmabandu et al. in Physical Review Letters 76, 2053 (1996)—no useful new laser sources resulted from this work. The wavelengths of the LWI output beams generated were ones that could all easily be obtained with the use of standard commercial lasers. In addition, the LWI output beam powers were very much lower than those of the auxiliary CW lasers that were required to pump the LWI devices. It is therefore small wonder that today one can find practically no mention of CW LWI sources in trade magazines (e.g. Laser Focus World) for the photonics and optoelectronics industries.
The fact that no practical lasers resulted from early intensive efforts to develop coherent light sources based upon dressed-atom gaseous media might relate in part to the somewhat restrictive pumping schemes that were employed to excite the coherently phased atoms in these LWIs. However, clear demonstrations of the considerable advantages dressed-atom media can offer for coherent light generation were shown in two later experimental studies (Merriam et al., IEEE Journal of Selected Topics in Quantum Electronics 5, 1502 (1999); Merriam et al., Physical Review Letters 84, 5308 (2000)). In each of these experiments almost complete conversion of pulses of ultraviolet laser light into pulses of vacuum ultraviolet (VUV) coherent light was achieved through resonant nonlinear mixing occurring in a gas of coherently phased lead (Pb) atoms. Although these two studies involved only pulsed coherent light beams, they vividly demonstrated that significant extensions in the wavelength ranges of coherent light sources can be efficiently attained via nonlinear mixing of resonantly-tuned laser beams applied to dressed-atom gaseous media.
As has here been implied, the laser field is by now very mature. However, it still abounds with interesting ideas for new lasers which might prove to be of great utility if they could in fact be realized. One of these ideas centers on a laser that would utilize the fundamental resonance transitions on which intense narrow-band light is emitted in commercial low-pressure gas-discharge lamps, e.g. Na vapor street lamps that emit mostly on the “D” lines at 5890  and 5896  or Hg vapor fluorescent bulbs that emit primary radiation largely at 1849.5  and 2536.6 . The signature feature of such lamps is the high efficiency with which the narrow-band light is produced. However, as yet no way has been found to tap this efficient source of narrow-band light to make a practicable laser. The main obstacle here has always been that the lower states of the resonance transitions are the ground states of the light-emitting atoms or ions. Hence the gas in such lamps is always very strongly absorbing at the wavelengths of the emitted resonance fluorescence, even when the lamps are turned on. In addition, it would be extremely difficult to maintain population inversions on such resonance transitions.
In the present invention, both method and apparatus are provided by which major deficiencies which were present in both earlier mentioned CW LWI experimental demonstrations can be overcome. It is herein disclosed how both CW and pulsed coherent light beams can be efficiently generated through the use of a novel dressed-atom-gas pumping scheme, with power for the pumping scheme being entirely provided by a simple continuously operating gaseous electrical discharge. In the disclosed invention there is no intrinsic need for additional lasers to supply pump power, as was required in the above mentioned early LWI efforts. It is, therefore, a main object of the present invention to disclose a new type of gaseous optical gain medium which can provide the basis for a family of useful and practicable coherent light amplifiers and oscillators that operate without population inversions being present on any optical transitions of the atoms, ions, or molecules comprised by the medium. It is another object of the present invention to disclose a new type of gaseous optical gain medium in which amplification results from a novel pumping scheme involving nonlinear excitation of a dressed-atom gas, with the actual power used in pumping not having to be supplied by any auxiliary laser beams applied to the medium. It is a further object of the present invention to provide both method and apparatus for a useful and practicable gas phase device that efficiently generates coherent light on certain resonance line transitions at which light is efficiently produced in low-pressure fluorescent lamps. It will be apparent from a reading of this description how the invention achieves these and other objects, which objects will become apparent as this description proceeds.